(a) Field of the Invention
The present invention relates to a shift control method of an automatic transmission, and a system thereof. More particularly, the present invention relates to a method and a system for releasing hydraulic pressure of an automatic transmission that improve shift responsiveness and stability of hydraulic pressure control as a consequence of quickly supplying, to a pressure control valve as feedback pressure, a part of operating pressure that is supplied from the pressure control valve to an off-going element in a case of releasing the off-going element.
(b) Description of the Related Art
An automatic transmission controls a hydraulic pressure by operating at least one solenoid valve according to a driving state, such as a vehicle speed and a throttle opening, in order to perform shifting to a target shift speed.
In a case in which the shifting to the target shift speed is performed, an automatic transmission has an off-going element that changes from an engaged state to a released state and an on-coming element that changes from a released state to an engaged state. Releasing an off-going element and engaging an on-coming element is realized by controlling hydraulic pressure supplied to respective elements.
In addition, the control of the hydraulic pressure supplied to the off-going and on-coming elements is performed by operating a control duty and controlling the solenoid valve according to the control duty.
That is, the solenoid valve supplies hydraulic pressure corresponding to the control duty to a pressure control valve, and the pressure control valve controls the hydraulic pressure transmitted from the solenoid valve and supplies it to the on-coming element or the off-going elements.
In addition, a pressure switch valve is used such that the pressure control valve does not operate too sensitively. In this case, control of the off-going element begins after hydraulic lines in the pressure switch valve are converted by the hydraulic pressure supplied to the pressure switch valve through the pressure control valve. Therefore, release response may be retarded and off-going pressure may change as a consequence of local change in flux in a case that the hydraulic lines in the pressure switch valve are converted.
If the release of the hydraulic pressure is retarded, release control of the off-going element in shift process may be retarded and thereby shift response may also be retarded. In addition, if undershoot of the off-going pressure occurs, the off-going element may be released instantaneously and blow-up of turbine speed may occur.
As shown in FIG. 6, if a shift signal is detected, a transmission control unit reduces control duty of a solenoid valve to release duty DR by a constant slope and maintains the control duty as the release duty DR. After that, the transmission control unit reduces the control duty of the solenoid valve to 0%.
At this time, release response may be retarded and the hydraulic pressure may change unstably because of conversion of the hydraulic lines in the pressure switch.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.